1. Field of the Invention
The present invention relates to an exhaust system for a watercraft, and more particularly to a catalytic exhaust system.
2. Description of Related Art
Personal watercraft have become very popular in recent years. This type of watercraft is quite sporting in nature and carries one or more riders. A relatively small hull of the personal watercraft commonly defines a riders' area above an engine compartment. A two-cycle internal combustion engine frequently powers a jet propulsion unit which propels the watercraft. The engine lies within the engine compartment in front of a tunnel formed on the underside of the watercraft hull. The jet propulsion unit is located within the tunnel and is driven by a drive shaft. The drive shaft usually extends between the engine and the jet propulsion device, through a bulkhead of the hull tunnel.
An exhaust system of the personal watercraft discharges engine exhaust to the atmosphere either through or close to the body of water in which the watercraft is operating. Although submerged discharge of engine exhaust silences exhaust noise, environmental concerns arise. These concerns are particularly acute in connection with two-cycle engines because engine exhaust from two-cycle engines often contains lubricants and other hydrocarbons.
Such environmental concerns have raised a desire to minimize exhaustion of hydrocarbons and other exhaust byproducts (e.g., carbon monoxide and oxides of nitrogen), and thus reduce pollution of the atmosphere and the body of water in which the watercraft is operated. In response to the increased concerns regarding exhaust emissions, some personal watercraft engines recently have become equipped with a catalyst to convert exhaust byproducts to harmless gases.
Catalysts must operate at a relatively high temperature in order to produce the necessary thermal reaction and burning of the exhaust byproducts. A catalytic device thus desirably operates within a specific range of temperature so as to effectively and efficiently convert engine exhaust into generally harmless gases.
Some prior exhaust systems have employed a cooling jacket about the catalytic device to maintain the catalytic device within the desired temperature range. In some systems, at least a portion of the cooling water also is introduced into the exhaust system not only to further cool and silence the exhaust gases, but also to assist the discharge of exhaust gases. The added water to the exhaust system, however, gives rise to possible damage to the catalyst.
In addition, some personal watercraft engines recently have become equipped with electronic control units that adjust the fuel/air charge delivered to the engine depending upon the operating condition of the engine. The ECU receives signal data from a variety of engine sensors and then controls engine operation in order to optimize performance, while minimizing hydrocarbon emissions.
An oxygen sensor is one of the main controlling sensors in the control system. The oxygen sensor monitors oxygen content in the exhaust. The amount of oxygen in the exhaust indicates the richness (low oxygen content) or leanness (high oxygen content) of the fuel/air charge. Based upon this information, the ECU alters the concentration of fuel in the air fuel charge to control emissions. The oxygen sensor probe often is located in a probe aperture in which exhaust gases collect for sampling. The pressure pulses or waves within the exhaust system produce a flow of exhaust gas into and out of the aperture for sampling purposes.
The cooling water introduced into the exhaust system, however, often interferes with the collection of exhaust gases in the exhaust probe aperture. The cooling water also commonly invades the aperture, thereby preventing an influx of exhaust gases. The oxygen sensor consequently provides imprecise measurements of the oxygen content in the exhaust gas flow.